Artesania rack

It is on the bottom but there is more than enough space above. 
most possible it is microphonics from tube. Given that the tube is microphonic,why the vibration travels from krion platform to external structure to internal structure of artesania and finally to preamp and tube? Is it normal to happen?

Isolation from a rack (or platform, or feet) is never 100% effective. There are always tradeoffs with what kinds of energy it’s designed to handle, where the energy is allowed to come from, and how the rack handles it (i.e. route it elsewhere or try to dissipate). Tapping the rack is generally NOT a good simulation of the kinds of energy your rack encounters during playback.

What you describe, as @lewm pointed out, sounds like a classic microphonic tube. It can be very tricky to select quiet tubes in a phono stage - besides microphonics they may have too much hisssss (noise floor) or make intermittent whistling sounds, rustling leaves, pings, pops, etc. Tapping around is good for finding out microphonic tubes, at least. When you have a microphonic tube, I have found that NO amount of fancy platforms or tube dampers will help enough. You should look for a quieter tube (or a quieter component!). You also have to get used to the fact that all tubes are microphonic by nature, and learn what is an acceptable amount in your phono and preamp slots. I once had a preamp with so many 6H30 with so much net gain (and downstream high sensitivity too), that when playing loud I could hear the trademark 6H30 "tinnnnng" ringing from the tubes between notes or at the start of pauses. Even with very carefully selected tubes. THAT was not an acceptable level of microphony!

6SN7 in slots that see a large amount of downstream gain are particularly diabolical, since they have relatively large plates for such high gain, and the older 1940s tube internals are all kinda loose at this point. 

Thank you for your answers. Thank you all, especially mulveling.